Forsterite glass-ceramics of high crystallinity and chrome content

ABSTRACT

A composition for a glass-ceramic material that contains a crystallinity of at least about 30% by weight of forsterite components at a liquidus temperature of about 1525 ° C. or below. The glass-ceramic has a composition, in weight percent on an oxide basis, consisting essentially of about: 40-60% SiO 2 ; 10-25% Al 2 O 3 ; 18-30% MgO; 3-10% Na 2 O; 0-10% K 2 O; &gt;5-15% TiO 2 . The invention further comprises a method for achieving high crystalline yield at such a low liquidus with increased solubility of high levels of chromium ions. The glass-ceramics can be used in drawing optical fibers and as gain media in amplifier and laser devices for near infrared wavelengths.

RELATED APPLICATIONS

[0001] U.S. patent application Ser. No. 09/686,418, entitled TRANSPARENTFORSTERITE GLASS-CERAMICS, filed on Oct. 11, 2000, in the name of GeorgeH. Beall, claiming priority to U.S. Provisional Application No.60/174,012 filed on Oct. 18, 1999.

[0002] U.S. patent application Ser. No. 09/686,564, entitledTRANSITION-METAL GLASS-CERAMIC GAIN MEDIA, filed on Oct. 11, 2000, inthe name of George H. Beall, Nicholas F. Borrelli, Eric J. Mozdy, andLinda R. Pinckney, claiming priority to U.S. Provisional Application No.60/160,053 filed on Oct. 18, 1999.

FIELD OF INVENTION

[0003] The present invention relates generally to glass-ceramicscontaining forsterite (Mg₂SiO₄) as the major crystalline phase. Moreparticularly, the glass-ceramics have a small crystal size to make theglass-ceramic material optically transparent and are doped with chrome(Cr) at relatively high levels, which is useful as gain media, inoptical amplifiers and/or laser pumps. The term “gain media” refers toan optical component that produces optical fluorescence and is capableof amplifying an optical signal in the same wavelength range as theoptical fluorescence. The invention also relates to a more formableglass-ceramic composition that is useful for drawing optical fibers.

BACKGROUND

[0004] Recently, researchers have concentrated much effort to developtransparent glass-ceramics as hosts for transition metal ions.Transition metals have been used as optically active dopants incrystalline hosts because they fluoresce in the near infrared (˜1000 nmto ˜1500 nm) region. Given the useful wavelength range and relativelywide bandwidth of many transition-metal dopants, much interest hasarisen for their use in optical telecommunication applications. Thecurrent optical telecommunication. medium is glass-based optical fiber.Inclusion of transition metal dopants into glasses, however, hasunfortunately not produced fluorescence performances as good as incrystalline materials. The performance of transition metal ions tends todegrade in amorphous hosts, where the crystal field strength is muchsmaller than in even single-crystal hosts.

[0005] Suitable glass-ceramic hosts, therefore, must be tailored suchthat transition elements will preferentially partition into the crystalphase. Some of these glass-ceramics have come from compositions such asthose discussed in co-pending U.S. patent application Ser. No.09/686,418, entitled TRANSPARENT FORSTERITE GLASS-CERAMICS, by George H.Beall, which relates to a family of glass compositions based in theK₂O—MgO—Al₂O₃—SiO₂ system, or in co-pending U.S. patent application Ser.No. 09/686,564, entitled TRANSITION-METAL GLASS-CERAMIC GAIN MEDIA, byGeorge H. Beall et al., which relates to transition-metal-dopedglass-ceramic materials used as gain media or pump laser fiber inoptical amplifiers and lasing mechanisms. The entire contents of both ofthese applications are incorporated herein by reference.

[0006] Glass-ceramics are polycrystalline materials formed by acontrolled crystallization of a precursor glass. In general, the methodfor producing such glass-ceramics customarily involves three fundamentalsteps: first, melting a glass-forming batch containing the selectedmetallic oxides; second, cooling the melt to a temperature at leastbelow its transformation range, while simultaneously forming a glassbody of a desired geometry; and third, heating the glass body to atemperature above the transformation range of the glass in a controlledmanner to generate crystals in situ. To develop nuclei in the glass, theglass will be heated initially to a temperature within or somewhat abovethe transformation range for a period of time. Thereafter, thetemperature will be raised to levels approaching, or even exceeding, thesoftening point of the glass to grow crystals from the nuclei. Theresulting crystals are typically uniformly distributed and fine-grained.Internal nucleation permits glass-ceramics to have favorable qualitiessuch as a very narrow distribution of particle size and a highly uniformdispersion of crystals throughout the glass host.

[0007] Transparent glass-ceramics are known in the art, with the classicstudy relating to transparency being authored by G. H. Beall and D. A.Duke in “Transparent Glass Ceramics,” Journal of Material Science, 4,pp. 340-352 (1969). Glass-ceramic bodies will display transparency tothe human eye when the crystals present therein are considerably smallerthan the wavelength of visible light. In other words, transparencytypically results from crystals less than 50 nm—preferably as low as 10nm—in size. Transparency in glass-ceramics, alternatively, can also beproduced with crystals larger than 50 nm if the crystal birefringenceand the index of refraction mismatch between the crystal phase and theglassy phase are both low. Transparent glass-ceramics, doped withtransition elements can combine the optical efficiency of crystals withthe flexibility of the forming of glass. For example, both bulk (planarsubstrates) and fiber forms can be fabricated from these glass-ceramics.

[0008] Forsterite is an orthosilicate with two distinct octahedralsites, both occupied by Mg²⁺, and one tetrahedral site occupied by Si⁴⁺.All three of these cation sites are highly distorted. The octahedralsites have mirror and inversion symmetries and the tetrahedral site ispyramidally distorted. It has been shown that chromium ions can enterthe forsterite structure as Cr³⁺ in the octahedral sites, and as Cr⁴⁺ inthe tetrahedral sites. The Cr⁴⁺ ion has further been identified as thekey lasing ion in single crystals responsible for the major portion ofluminescence over the wide band extending from about 900 nm to about1400 nm and centered at about 1175 nm. (A shoulder on the band near 1000nm is attributed to Cr³⁺ ions).

[0009] Chromium-doped forsterite, a transition-metal-silicate crystalspecies, has demonstrated the ability to produce optical gain over abroad portion of the near infrared spectrum and has been fabricated assingle-crystal tunable or femtosecond lasers. In the late 1980s, it wasdiscovered that single crystals of chromium-doped forsterite could beused as a laser material in the 1210 nm to 1260 nm region. Further work,determined that the active ion was Cr⁴⁺, a rare valence state ofchromium, and that strong luminescence and tunable laser action could beproduced in the broad spectral region from about 1100 nm to about 1400nm, and perhaps even deeper into the infrared.

[0010] In the past, however, the maximum gain (complete populationinversion) for a material with about 25% crystalline forsteriteparticles was calculated, using published optical constants forforsterite, to be about only 240 dB/m. To increase the overallfluorescence within forsterite-containing glass-ceramic materials, agreater crystalline yield of forsterite needs to be achieved. Thepresent invention provides a method and glass composition that satisfiesthis need.

SUMMARY OF THE INVENTION

[0011] The present invention resides in part in transparentglass-ceramics that have a level of nanocrystallinity of at least about30% forsterite components by weight when at a relatively low liquidustemperature of about 1525-1500° C. or less. This is a higher yield offorsterite component and crystals than was previously achievable, on asustainable basis, at such relatively low temperatures. The predominantforsterite crystal phase in the glass-ceramic is doped with chromium atlevels higher than that which was previously practical to performforming operations such as drawing optical fibers. In part, the key toimprovements in formability with greater crystallinity of forsteriteinvolves the addition of Na₂O as a major ingredient (over about 3% byweight), coupled with increased levels of titania (≧5% by weight) in anoriginal glass composition. The effect of increased amounts of Na₂O, inreplacing K₂O amounts, is to lower the liquidus temperature offorsterite at a given level of theoretically attainable forsterite inthe original glass composition. Moreover, increased sodium levels leadto higher allowable MgO levels, which in turn, increase the solubilityof Cr⁴⁺ ions in the glass. As a result, more Cr⁴⁺ ions are thenavailable to be incorporated into forsterite nanocrystals for greaterluminescence. In short, the invention provides better physicalflexibility in forming glass-ceramic objects, and better overallfluorescence and performance in optical gain, due to greatercrystallinity.

[0012] The glass-ceramics of the present invention have a glasscomposition, in weight percent on an oxide basis of about 40% to 60%SiO₂; 10% to 25% Al₂O₃; 18% to 30% MgO; 3% to 10% Na₂O; 0% to 10%K₂O; >5% to 15% TiO₂. Preferably, the composition consists essentiallyof about 43% to 55% SiO₂; 11% to 16% Al₂O₃; 20% to 26% MgO; 3.5% to 6.5%Na₂O; 3.0% to <8.0% K₂O; 5.5% to 9.0% TiO₂. To obtain optical activity,i.e., fluorescence, over the infrared telecommunications transmissionwavelength range of about 900 nm to about 1400 nm, the presentforsterite glass-ceramics are doped with up to about 1.3% chromium oxideby weight, and preferably with about 0.05% to about 0.75% chromiumoxide.

[0013] The present invention also encompasses a method of dissolving atleast 30% by weight of forsterite components in a glass-ceramic. Themethod comprises providing a R₂O—MgO—Al₂O₃—SiO₂ glass composition,wherein R is an alkali ion, containing, in weight percent, at leastabout 3% of Na₂O coupled with greater than 5% of TiO₂, and melting theglass at a temperature between about 1575° C. to about 1650° C.Preferably the temperature ranges from about 1590° C. to about 1630° C.Then, heat treating the glass according to a ceramming schedule toprecipiate crystals, and achieve at least 30% by weight of forsteritecomponent in the glass-ceramic at a liquidus temperature of about 1525°C. or below.

[0014] The present invention further includes an optical fiber and/or again medium comprising a transparent glass-ceramic containing acrystallinity of at least about 30% by weight of forsterite componentsat a liquidus temperature of about 1525° C.±5° C. or less.

[0015] Additional features and advantages of the invention will bedescribed in the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1A is a phase diagram for the K₂O—MgO—Al₂O₃—SiO₂ system.

[0017]FIG. 1B is a phase diagram for the Na₂O—MgO—Al₂O₃—SiO₂ system.

[0018]FIG. 2 compares the fluorescence intensity of two kinds offorsterite-crystal-containing glass-ceramics—one with Na₂O and anotherNa₂O-free—over a spectrum from about 900 nm to about 1400 nm.

[0019]FIG. 3A is a diagram showing absorption spectra at roomtemperature of a higher crystallinity Na₂O-containing glass-ceramicdoped with Cr⁴⁺ and its precursor glass, according to the presentinvention.

[0020]FIG. 3B is a diagram showing absorption spectra at roomtemperature of lower crystallinity Na₂O-free glass-ceramics doped withCr⁴⁺ and its precursor glass.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention is based in part on the discovery of amethod to increase the overall crystallinity in a glass-ceramic, whereinat least 30% by weight of forsterite component is dissolved at arelatively low liquidus temperature of about 1525° C. or below. Themethod comprises providing a R₂O—MgO—Al₂O₃—SiO₂ glass composition,wherein R is an alkali ion, containing, in weight percent, at leastabout 3% of Na₂O coupled with greater than 5% of TiO₂, and melting theglass at a temperature between about 1575° C. to about 1650° C., orpreferably about 1580° C. to about 1635° C. Subsequently heat treating(ceramming) the glass to form a glass-ceramic. The R₂O—Al₂O₃—SiO₂ glasscomposition belongs to a family of compositions that can produce glassesof excellent stability and which can produce, when cerammed,substantially transparent glass-ceramics containing forsterite as thepredominant crystal phase.

[0022] To produce a fine-grained glass-ceramic based on forsterite, itwas necessary to produce a glass with amorphous phase separation, whereone of the phases is highly enriched with MgO and the other is rich inglass formers, upon cooling or subsequent heating. This is becauseforsterite itself melts at 1890° C. (±20° C.) and does not form a glasseven when rapidly cooled. Merely adding glass formers, like SiO₂, Al₂O₃,or B₂O₃, were not helpful since they only produced other phases likeenstatite (MgSiO₃), corderite (Mg₂Al₄Si₅O₁₈), or various otherMg-phases. The challenge, then, was to create a stable glass from whichforsterite, and not the more siliceous Mg-rich crystals could form.

[0023] Previously, in the K₂O—MgO—Al₂O₃—SiO₂ system, glasses containingforsterite components were formulated. These glasses, however, never hadmore than about 25 wt % maximum crystal phase under reasonablemanufacturing temperatures (≦˜1500° C.). The key clue to creating a morecrystalline, transparent glass-ceramics, via phase separation from aglass composition that is rich in forsterite-components, derives fromphase equilibria data in the K₂O—MgO—Al₂O₃—SiO₂ and analogousNa₂O—MgO—Al₂O₃—SiO₂ systems, which is shown, respectively, in the phasediagrams of FIGS. 1A and 1B. In the center of the forsterite liquidusfield in the Na₂O—MgO—Al₂O₃—SiO₂ system of FIG. 1B, liquids saturatedwith about 45% forsterite by weight tend to crystallize from the melt atabout 1500° C. In the analogous K₂O—MgO—Al₂O₃—SiO₂ system, at 1500° C.,only about 25% forsterite component is soluble in the melt in the centerof the forsterite liquidus field. This means that larger quantities ofthe forsterite component can be dissolved in NaAlSi₂O₆ glasses than inKAlSi₂O₆ glasses at the same liquidus temperature. Hence, aglass-ceramic containing more forsterite should be possible in the sodicsystem, as compared to the potassic system, without increasing thetemperature. This feature saves costs and is advantageous formanufacturing conditions used in operations like fiber drawing. Thecompositions of the present invention detail a way to produce highforsterite crystallinity at relatively low liquidi.

[0024] The sodic family of glass compositions, in weight percent on anoxide basis, consists essentially of about 40% to 60% SiO₂; 10% to 25%Al₂O₃; 18% to 30% MgO; 3% to 10% Na₂O; 0% to 10% K₂O; >5% to 15% TiO₂.Preferably, the compositions consist essentially of about 43% to 55%SiO₂; 11% to 16% Al₂O₃; 20% to 26% MgO; 3.5% to 6.5% Na₂O; 3.0% to <8.0%K₂O; 5.5% to 9.0% TiO₂. Table 1 presents a few particular examples ofglass compositions, which illustrate the parameters of this family, withthe amount of each oxide expressed in terms of total weight percent. Ofthese examples, it was observed that the samples with mixed alkaliexperienced good nucleation (with between more than about 5.1 wt % toless than about 10 wt % of TiO₂), produced more forsterite, and formedstable glasses at liquidi of 1500° C. or below. A lower liquidustemperature allows fiber to be drawn at lower temperatures than usual,minimizes mixing (contamination) between materials of the fiber core andits cladding during drawing, and makes the highly Cr-doped glass lessprone to uncontrolled devitrification. When Na₂O and K₂O are bothpresent in mixed alkali samples, their content should be preferably inabout a 1:1 molar ratio. Other glass forming oxides could also beincluded, such as possibly up to about 20 wt % of GeO₂, with acomparable molar percentage reduction of SiO₂. Trace to minor amounts ofother alkali oxides, such as Li₂O, Rb₂O and Cs₂O, as glass softeners arepossible.

[0025] Fluorescence in the inventive forsterite glass-ceramic is inducedby the addition of up to about 1.3 wt % Cr₂O₃, with preferred rangesfrom about 0.5-1.0 wt %, or more preferably within 0.55-0.7-0.8 wt % ofchromium oxide (Cr⁴⁺ or Cr³⁺), to the parent glass. Other transitionmetal ions, including but not limited to Ni²⁺, V³⁺, Co²⁺, Cu²⁺, Cu₁₊,Mn²⁺, Fe²⁺, and Ti³⁺, also can be used as dopants in like amounts. Thecrystal structure of forsterite, as mentioned before, provides bothtetrahedeal and octahedral cation sites of appropriate size to housetransition metal cations. In particular, Cr⁴⁺ ions are incorporatedtetrahedrally, and Cr³⁺, Ni²⁺, Co²⁺, Cu²⁺, and Mn²⁺ ions areincorporated octahedrally into the respective crystal sites. As known inthe optics and laser arts, crystals with tetrahedrally coordinated Cr⁴⁺ions provide unique optical characteristics. Hence, the presentinventive forsterite glass-ceramics are suitable for opticaltelecommunication uses in devices like optical fiber and gain media foramplifiers and pump lasers.

[0026]FIG. 2 shows the qualitative, relative fluorescence of aNa₂O-containing, Cr4+-doped forsterite glass-ceramic (Line A) comparedwith a Na₂O-free glass-ceramic (Line B). Both glass-ceramics have anemission bandwidth from about 900 nm to about 1440 nm, and are bothcentered at a wavelength of about 1150 nm, when pumped with a 800 nmlaser to excite Cr⁴⁺ ions doped within. The Na₂O-containing compositionhas a higher fluorescence intensity, indicating the presence of agreater concentration of Cr⁴⁺ ions, than the other glass. A morespecific picture regarding Cr⁴⁺ content is presented in FIGS. 3A and 3B.FIG. 3A shows the respective absorption spectra at room temperature of apiece of bulk higher crystallinity Na₂O-containing glass-ceramic dopedwith Cr⁴⁺, according an embodiment of the present invention, compared toa “virgin” Cr³⁺-containing glass before ceramming. FIG. 3B shows ananalogous absorption spectra at room temperature of the same precursorCr³⁺-containing glass and a lower crystallinity, potassic, Na₂O-freeglass-ceramic fiber doped with Cr⁴⁺. Looking at FIGS. 3A and 3Btogether, clearly evident is the increase in absorption capacity in theNa₂O-containing glass-ceramic vis-à-vis the Na₂O-free glass. This meansthat the Cr⁴⁺ content has greatly increased relative to the latter,potassic composition.

[0027] To produce a good glass-ceramic gain medium, with higher thanaverage fluorescence intensity, like that shown in FIG. 2, thecrystalline phase needs to be doped with high levels of chromium. This,however, presents a problem. Formerly, a glass composition that washighly doped with chrornium ions tended to devitrify uncontrollably.This phenomenon makes a high chromium-content glass impractical ordifficult to use for most manufacturing applications such as fiberdrawing. The present invention solves this problem by providing glasscompositions with lower liquidi—making them less likely todevitrify—that are more readily capable of being fiberized at lowertemperatures, in contrast to potassic compositions with higher liquidi.Without being held to any particular theory, we believe that thisphenomenon is due in part directly to greater solubility of chromium inthe sodic and mixed alkali system(s)—sodium-potassium. A secondadvantage of a relatively low liquidus temperature glass of the presentinvention is that for fiberization at lower working temperatures thecore and cladding materials do not mix as readily. Hence, lessening thechance for core-cladding contamination or transmigration.

[0028] A higher crystalline yield of forsterite in the glass-ceramic isanother result of the presence of a relatively high level of sodium, andshould increase luminescent intensity and overall quantum efficiency,approaching those values for single-crystals. The higher sodium levelspermit a relative increase in the level of MgO in the glass. Higher MgOlevels, in addition to promoting forsterite formation, makes the glasschemically more alkaline or basic. The ratio of chromium 4+ ions to thetotal chromium content (Cr⁴⁺/Cr) in the glass increases with increasedMgO content and alkalinity. Higher chromium levels allow more Cr⁴⁺ to beavailable for incorporation into the forsterite nanocrystals, thusfurther improving luminescence. Also, increased crystallinity decreasesthe space between crystals and promotes Cr⁴⁺ ions left in the residualglass phase to migrate into the crystals during heat treatment.

[0029] Thus, a possible additional explanation for the increasedluminescence and performance observed in FIG. 2, could be the increasedalkalinity of the glass, which allows for greater magnesium solubility.Experimental data from both fluorescence and absorbance measurementsindicate that the increase of Cr⁴⁺ content is greater than would benormally expected from a mere increase in crystalline content. Thepresent glass compositions, having an increase in crystallinity of up toabout 50%, produced a Cr⁺⁴ absorbance that appeared to be four timesgreater than that found in prior compositions. See, FIGS. 3A and 3B.

[0030] The level of alkalinity in these glasses can be estimated by thesimple ratio: (MgO+R₂O)/(Al₂O₃+SiO₂), where R is an alkali ion. It isbelieved that high valence states of chromium, in particular Cr⁴⁺ andCr⁶⁺, increase with greater alkalinity. Thus, the present compositionspermit a higher level of Cr₂O₃ to be dissolved in the glass. This notonly does not cause the glass to undergo uncontrolled devitrification,but rather, contributes to flexible forming of high Cr⁴⁺-doped opticalfibers, since glass-ceramic materials are glass-based, they can beformed into any shape prior to nucleation and can be readily spliced tosilica glass fibers. A transition metal doped glass-ceramic gain mediais advantageous in that it can provide gain across every wavelengthconceivable of interest in telecommunications today.

EXAMPLES

[0031] In a conventional furnace operating at temperatures of at least1400° C., preferably about 1580-1620° C., the compositional examples ofTable 1 were produced by melting well-mixed batch compounds in platinumcrucibles for about 4 to 16 hours. Some melts were then poured as free“patties” and transferred to an annealer operating at about 550-600° C.Other melt samples, with high chromium levels of about 0.4 wt. % to over1 wt. %, were either quenched with steel plungers or metallic rollers,or drawn as a cane (rod) or pulled as glass fibers directly from thecrucible. Table 1 also presents the ceramming schedule in degreesCelsius and hours for each example, as well as the crystal phase(s)observed in each resultant glass-ceramic. The glass patties weresubjected to a ceramming cycle, wherein the patties were first heated toa temperature within the range of about 600-800° C. for a period of timesufficient to generate nuclei, usually between about 4-12 hours. Second,the nucleated glass patties were then again heat-treated at atemperature within the range of about 750-950° C. for a period of timesufficient to grow crystals from the nuclei, typically between about 1-4hours.

[0032] The compositional examples were then examined using x-raydiffraction to identify their crystalline phases. All of the examplesproduced a predominant phase of forsterite crystals. The inventivecompositions are self-nucleating because of liquid-liquid phaseseparation. Phase separation in the sodic system, however, is moredifficult to achieve, unless large amount of titantia (TiO₂ over about10 wt %) is present. Typically, in the sodic system glasses tends toproduce relatively large-sized crystals. TiO₂ functions as a nucleatingagent to control crystal size. Due to the presence of high levels ofTiO₂, some examples also included either faint traces or minor levels ofrutile percipitate. Yet, controlled use of optimized amounts of TiO₂,within a preferred range of about 5.7% to about 8.7% or 9.0% by weight,can either minimize greatly or eliminate this issue—as long as theamount and size (˜10 nm) of rutile crystals present are small enough notto affect transparency in the near infrared region. Mixed Na₂O and K₂Osamples showed the best results, with good nucleation and phaseseparation while using less than about 8.6 wt % TiO₂. Normally, highTiO₂ content produces opalescent to opaque glasses. The presentcompositions, although relatively high in TiO₂ content, are transparentto translucent. Titania in this situation helps produce a finer crystalsize and improves transparency and/or translucency.

[0033] In the present inventive glass-ceramic, transparency ortranslucency is a function of the microstructure, which is a function ofthe composition and heat treatment. The microstructure of the inventiveglass-ceramics contain forsterite nanocrystals of about 10 nm to about50 or 60 nm in size, with a preferred size of under about 35 nm or about30 nm. The present compositions enable one to create a glass-ceramicwith a ≧30% yield of forsterite crystals by weight while at a relativelylow liquidus temperature, within the range of about 1475° C. to about1550° C., preferred range of about 1493±5° C. to about 1520±5° C.Preferably, the amount of crystallinity is about 35% or 40% to about 55%by weight.

[0034] Potential applications and devices that make use of the presentinventive compositions include femtosecond and tunable lasers, widebandwidth optical fiber amplifiers, and regenerative amplifiers in thenear infrared wavelengths. Other possible applications and devices aredescribed in detail in co-pending U.S. patent application Ser. No.09/686,564, incorporated herein by reference. TABLE 1 ForsteriteGlass-Ceramic Compositions EXAMPLE No. (wt %) OXIDES Ex. 1 Ex. 2 Ex. 3Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 SiO₂ 47.6 47.4 48.8 47.9 51.1 44.1 47.946.2 Al₂O₃ 13.3 13.3 11.3 13.5 13.1 17.5 13.4 12.0 MgO 21.6 21.4 22.121.8 18.7 21.4 21.7 24.8 Na₂O 8.2 5.4 7.0 4.1 7.9 10.6 4.1 7.3 K₂O — 4.1— 6.2 — — 6.2 — TiO₂ 9.1 8.2 10.7 6.4 9.1 6.2 6.3 9.0 CrO₃ 0.2 0.2 0.20.2 0.2 0.2 0.4 0.6 Glass Quality Green, clear Green, clear Green, clearGreen, clear Green, clear Green, clear Dark Green, Very clear Dark GreenHeat Treatment 750 @ 8 750 @ 8 700 @ 8 700 @ 8 750 @ 8 750 @ 8 700 @ 8700 @ 8 Temp. (° C.) @ hrs. 850 @ 2 850 @ 2 850 @ 2 850 @ 4 850 @ 2 900@ 2 850 @ 4 850 @ 4 Glass-Ceramic Brown Brown Brown Greenish Olive BrownGreenish Dark Brown X-ray diffraction Forsterite, Forsterite,Forsterite, Forsterite Forsterite, Forsterite, Forsterite Forsterite,Crystal phase(s) Minor rutile Faint rutile rutile Minor enstatiteCordierite, Minor rutile Cristobolite Liquidus (° C.) 1450 1500 — 1500 —— 1500 —

[0035] Although the present invention has been described by way ofexamples, those skilled in the art will understand that the invention isnot limited to the embodiments specifically disclosed, and that variousmodification and variations can be made without departing from thespirit and scope of the invention. Hence, unless changes otherwisedepart from the scope of the invention as defined by the followingclaims, they should be construed as included herein.

We claim:
 1. A transparent glass-ceramic containing a predominantcrystal phase of forsterite, the glass-ceramic having a composition, inweight percent on an oxide basis, consisting essentially of about:40-60% SiO₂; 10-25% Al₂O₃; 18-30% MgO; 3-10% Na₂O; 0-10% K₂O; >5-15%TiO₂; and said glass-ceramic has a crystallinity of at least about 30%by weight of forsterite components at a liquidus temperature of about1525° C. or below.
 2. The glass-ceramic according to claim 1, whereinNa₂O and K₂O are both present in about a 1:1 molar ratio.
 3. Theglass-ceramic according to claim 1, wherein said TiO₂ content by weightin said composition is greater than about 6%, and less than about 9%. 4.The glass-ceramic according to claim 1, wherein said composition furtherincludes, in weight percent on an oxide basis, up to about 1.3% chromiumoxide.
 5. The glass-ceramic according to claim 4, wherein saidcomposition includes, in weight percent on an oxide basis, about 0.05%to about 0.75% chromium oxide.
 6. The glass-ceramic according to claim1, wherein said composition further includes, in weight percent on anoxide basis, up to about 20% GeO₂.
 7. The glass-ceramic according toclaim 1, wherein said composition includes a transition metal ionselected from the group consisting of Ni²⁺, V³⁺, Co²⁺, Cr⁴⁺, Cu²⁺, Cu¹⁺,Mn²⁺, Fe²⁺, and Ti³⁺.
 8. The glass-ceramic according to claim 1, whereinsaid crystallinity is about 35% or more by weight of forsteritecomponents.
 9. The glass-ceramic according to claim 1, wherein crystalsin the crystal phase have a size no larger than about 60 nm.
 10. Theglass-ceramic according to claim 1, wherein crystals in the crystalphase have a size between about 10 nm to about 35 nm.
 11. A transparentglass-ceramic with a crystallinity of at least about 30% by weight offorsterite components at a liquidus temperature of about 1525° C. orbelow, having a composition, in weight percent on an oxide basis,consisting essentially of about: 43-55% SiO₂; 11-16% Al₂O₃; 20-26% MgO;3.5-6.5% Na₂O; 3.0-8.0% K₂O; 5.5-9.0% TiO_(2.)
 12. The glass-ceramicaccording to claim 11, wherein Na₂O and K₂O are both present in about a1:1 molar ratio.
 13. The glass-ceramic according to claim 11, whereinsaid TiO₂ content by weight in said composition is greater than about6%, and less than about 9%.
 14. The glass-ceramic according to claim 11,wherein said composition further includes, in weight percent on an oxidebasis, up to about 1.3% chromium oxide.
 15. The glass-ceramic accordingto claim 14, wherein said composition includes, in weight percent on anoxide basis, about 0.05% to about 0.7% chromium oxide.
 16. Theglass-ceramic according to claim 11, wherein said composition furtherincludes, in weight percent on an oxide basis, up to about 20% GeO₂. 17.The glass-ceramic according to claim 11, wherein said compositionincludes a transition metal ion selected from the group consisting ofNi²⁺, V³⁺, Co²⁺, Cu²⁺, Cu¹⁺, Mn²⁺, Fe²⁺and Ti³⁺.
 18. The glass-ceramicaccording to claim 11, wherein said crystallinity is about 35% or moreby weight of forsterite components.
 19. The glass-ceramic according toclaim 11, wherein crystals in the crystal phase have a size no largerthan about 60 nm.
 20. The glass-ceramic according to claim 11, whereincrystals in the crystal phase have a size between about 10 nm to about35 nm.
 21. A method of dissolving at least 30% by weight of forsteritecomponent in a glass-ceramic, the method comprising: providing aR₂O—MgO—Al₂O₃—SiO₂ glass composition, wherein R is an alkali ion,containing, in weight percent, at least about 3% of Na₂O coupled withgreater than 5% of TiO₂; melting said glass at a temperature betweenabout 1575° C. to about 1650° C.
 22. The method according to claim 21,wherein said glass has a composition, in weight percent on an oxidebasis, consisting essentially of about: 40-60% SiO₂; 10-25% Al₂O₃;18-30% MgO; 3-10% Na₂O; 0-10% K₂O; >5-15% TiO₂.
 23. The method accordingto claim 21, further comprising achieving at least 30% by weight offorsterite component in said glass-ceramic at a liquidus temperature ofabout 1525° C. or below.
 24. The method according to claim 22, whereinNa₂O and K₂O are both present in about a 1:1 molar ratio.
 25. The methodaccording to claim 22, wherein said TiO₂ content by weight in saidcomposition is greater than about 6%, and less than about 9%.
 26. Themethod according to claim 22, wherein said composition further includes,in weight percent on an oxide basis, up to about 1.3% chromium oxide.27. The method according to claim 26, wherein said composition includes,in weight percent on an oxide basis, about 0.05% to about 0.7% chromiumoxide.
 28. The method according to claim 22, wherein said compositionfurther includes, in weight percent on an oxide basis, up to about 20%GeO₂.
 29. The method according to claim 22, wherein said compositionincludes a transition metal ion selected from the group consisting ofNi²⁺, V³⁺, Co²⁺, Cr⁴⁺, Cu²⁺, Cu¹⁺, Mn₂₊, Fe²⁺, and Ti³⁺.
 30. The methodaccording to claim 22, wherein said crystallinity is about 35% or moreby weight of forsterite components.
 31. The method according to claim22, wherein crystals in the crystal phase have a size no larger thanabout 60 nm.
 32. The method according to claim 22, wherein crystals inthe crystal phase have a size between about 10 nm to about 35 nm.
 33. Anoptical element selected from the group consisting of an optical fiber,a gain-medium, a laser, and an amplifier, said element comprising: atransparent glass-ceramic containing a crystallinity of at least about30% by weight of forsterite component at a liquidus temperature of about≦1525° C.±5° C. or below, the glass-ceramic having a composition,. inweight percent on an oxide basis, consisting essentially of about:40-60% SiO₂; 10-25% Al₂O₃; 18-30% MgO; 3-10% Na₂O; 0-10% K₂O; and >5-15%TiO₂.
 34. The optical element according to claim 33, wherein Na₂O andK₂O are both present in about a 1:1 molar ratio.
 35. The optical elementaccording to claim 33, wherein said TiO₂ content by weight in saidcomposition is greater than about 6%, and less than about 9%.
 36. Theoptical element according to claim 33, wherein said composition furtherincludes, in weight percent on an oxide basis, up to about 1.3% chromiumoxide.
 37. The optical element according to claim 36, wherein saidcomposition includes, in weight percent on an oxide basis, about 0.05%to about 0.7% chromium oxide.
 38. The optical element according to claim33, wherein said composition further includes, in weight percent on anoxide basis, up to about 20% GeO₂.
 39. The optical element according toclaim 33, wherein said composition includes a transition metal ionselected from the group consisting of Ni²⁺, V³⁺, Co²⁺, Cr⁴⁺, Cu²⁺, Cu¹⁺,Mn₂₊, Fe²⁺, and Ti³⁺.
 40. The optical element according to claim 33,wherein said crystallinity is about 35% or more by weight of forsteritecomponents.
 41. The optical element according to claim 33, whereincrystals in the crystal phase have a size no larger than about 50 mn.42. The optical element according to claim 33, wherein crystals in thecrystal phase have a size between about 10 nm to about 35 nm.